Jennifer Lopez turned 43 in style last night -- by partying on a giant boat with her much younger boyfriend.

The hot mama looked amazing in a tight, backless red dress as she danced the night away on a yacht in NYC.

By J.Lo's side all night: 25-year-old boyfriend Casper Smart.

The two looked very lovey-dovey while dancing together on the boat. And from the looks of the photos, Casper may have given Jennifer a new necklace as a present -- as she was seen holding onto the new accessory with a romantic look in her eye as they talked.

Casper also took to Twitter to celebrate his special lady's day, tweeting "(yelling) HAPPY BIRTHDAY TO THE MOST BEAUTIFUL BEAR IN THE WORLD!!!!!!!!!!!!!!! LOVE U.. ;) @JLo"